Translation:Yesterday was May fifth.
The Japanese sentence isn't making a comment about "May fifth"; it's making a comment about "yesterday".
"Yesterday" is the topic being commented on in this sentence.
きのう is the topic.
は is the topic marking particle.
The comment being made about this topic is that it "was May fifth".
五月五日でした is the comment.
In English, saying "May fifth was yesterday" would instead be saying something about May fifth.
- "きのうは五月五日でした" is stating what the date was yesterday.
- "Yesterday was May fifth" is stating what the date was yesterday.
- "May fifth was yesterday" would be stating how long ago it was May fifth.